Radially movable switch actuating rods which are engaged by the piston of a fluid cylinder as the piston approaches the end of its stroke are known in the art, but have deficiencies which have prevented the realization of a reliable mechanism. For example, such actuator devices are subject to an appreciable amount of side loading upon engagement with the piston. One way to accommodate such forces is to use a large diameter rod and closely support same in a guide bore. Since the rod is subject at one end to fluid pressure within the cylinder, high fluid pressure forces are encountered and a relatively stiff return spring must be utilized to prevent false actuations due to transient fluid pressures and the like. Friction effects of a large diameter rod are also of concern, affecting not only the reciprocating movement of the actuator rod but also any rotary movement within its guide bore. The rotary motion is useful in spreading the wear on the tip of the actuator plunger so that a longer life can be achieved with this part. Further, the larger sized rod requires a large seal which provides greater potential for friction and leakage problems.
An alternative arrangement is to use a slender actuator rod and some form of joint to eliminate or minimize side loads. Friction forces on the small rod are minimal and do not appreciably affect the linear or rotary movement of the rod. Because of this, a lighter return spring may also be used, reducing also the force at the plunger tip to alleviate to some extent the wear problem which occurs there.
One such prior art arrangement is shown in U.S. Pat. No. 3,661,053 wherein a slender rod actuator is combined with a ball plunger. The ball plunger eliminates side load effects on the rod and as it is free to rotate within its guide bore, distributes wear about the surface of the ball. This particular design, however, is subject to false actuations, possibly due to fouling of passages which transmit fluid cylinder pressures to prevent pressure unbalance upon the actuator and possibly due to clogging in the guide arrangement for the ball wherein the periphery of the ball is closely restrained. Further, in this design, a ball and matching ball race are required and this presents special design considerations for different size cylinders or where different cushion spears are used or where different pressure levels are encountered. Sizing the ball and race accordingly presents design considerations which are inconvenient and difficult to control on a mass-production basis.